I second the G-Tech. I talked to the original creator and decided to get one myself.
As in all devices of this type, how you measure and interpret is important. When the speed in a quarter mile is measured, they keep measuring after you actually pass the quarter mile mark and take an average of the before/after.
Things like this make it impossible to exactly duplicate results, but you are so close it really doesn't matter.
Just one thing, be sure you mount it according to instructions for the most accurate results.
Any other car is a compromise